MURRAY, Utah — Julia Evans loves her parents and her three brothers because they have fun together, support her and all want to be what Mormons call a "forever family."

The 15-year-old's own marriage and family seem far away now, but she talks about it sometimes with her friends in a Murray, Utah, congregation of The Church of Jesus Christ of Latter-day Saints. When she does, she tells them she wants an even larger family, with five or six children.

"I think it's exciting," she said Monday, hours before the release of a new study that found Mormons marry more than those in any other American faith and have the largest families.

The Pew Research Center interviewed more than 35,000 Americans for the study, a follow-up to its first religious landscape study in 2007. Due to the massive sample size, the study's margin for error was plus or minus 0.6 percent.

The new study found that the average number of children ever born to Mormons now between the ages of 40 and 59 is 3.4. The idea behind looking at that age group is to capture what the researchers called "completed fertility."

The LDS average was well above the next closest groups. The average number of children born to members of historically black Protestant churches was 2.5. Next were Catholics and evangelicals at 2.3.

The study's major overall takeaway is that the Christian share of the U.S. population is dropping, driven by declines in the number of mainline Protestants and Catholics. From Pew's first religious landscape study in 2007 to the newly released 2014 study, the percentage of Christians in the U.S. declined from 78.4 percent to 70.6 percent.

The study found that LDS Church numbers held steady. Its share of the rising U.S. population was 1.6 percent in 2014. Though that was down slightly from 1.7 percent in 2007, the difference was within the study's margin for error.

"It's also striking, and you see this in other national studies, that the percentage of Mormons doesn't really change, and that's interesting," said David Campbell, co-author of "American Grace: How Religion Divides and Unites Us."

Center core

The study's finding on Mormon families fits the church's teachings.

"It's our responsbility to have a family," Julia Evans said Monday, before sitting down to family dinner and Family Home Evening with her parents and Lincoln, 18, and Isaac, 9. Samuel, 19, is in Winston-Salem, North Carolina, serving as a missionary for the church.

Latter-day Saints believe parents are co-creators with God, and that families are central to God's plan.

The home is the center core of the LDS Church, and the most sacred relationships in the church are in the family, according to President Boyd K. Packer, president of the faith's Quorum of the Twelve Apostles.

Last month, Elder D. Todd Christofferson, also of the Twelve, said at the church's April general conference that "God ordained that men and women should marry and give birth to children, thereby creating, in partnership with God, the physical bodies that are key to the test of mortality and essential to eternal glory with Him."

Those teachings, and the belief that families can be united forever after this life, motivated Becky Evans, Julia's mother and a lifelong Mormon with three sisters of her own, to have a larger-than-average family.

"There are a lot of spirits that want to come down and have bodies," she said. "We wanted to provide a good family where they could be loved, and for as many as could be loved."

She and her husband, David, raised LDS in a family with five children, "wanted to have as many children as I could handle physically," she said, "and as many as we could handle financially and as many as we could bear emotionally. Four ended up being a good number for us."

The percentage of Mormons who are married declined from 2007 (71 percent), but the margin for error in the study's Mormon sample was plus or minus 4.9 percentage points.

One change among Mormons that was larger than the margin of error was an increase in the percentage who never have married, from 12 percent in 2007 to 19 percent in 2014.

Mormons surveyed also said they are raising more children than members of other religious groups. On average, Mormon adults of all ages reported they lived with 1.1 children as parents or guardians.

Retention rates

The study found that among Christian groups, historically black Protestant churches retain the highest percentage of their childhood members at 70 percent. Pew's data showed evangelicals were next at 65 percent, followed by Mormons at 64 percent.

The decline in the percentage of Christians in the U.S. population is mirrored by the rise of "nones," or those who say they have no affiliation with a church.

Of those who leave the historically black Protestant or evangelical traditions, more said they now identify with another religion now.

Among former Mormons, however, most said they now are "nones" — a total of 21 percent of adults raised LDS.

Of the Mormons surveyed, 69 percent said they, like each member of the Evans family, were raised in the faith. The rest of those who identified themselves as Mormons originally were Catholic (9 percent), unaffiliated (8 percent), Mainline Protestant (7 percent), evangelical (6 percent) or other (1 percent).

Campbell, also co-author of "Seeking the Promised Land: Mormons and American Politics," said that based on church growth data, he'd expect to see the number of Mormons as a percentage of the U.S. population rise. He said that might be a sign of an issue with convert retention.

"It's significant it's not dropping," he said, "but it's not growing anywhere near the extent that you might think."

Retention recently became a renewed focus of LDS missionary work. Elder Quentin L. Cook, also of the Twelve, mentioned retention in his April conference talk. In the talk's footnotes, he wrote, "Over the last 25 years, the actual number of members leaving the church has decreased and the church has almost doubled in size. The percentage leaving is greatly reduced."

Other numbers

The 2014 survey interviewed more than 650 people who identified themselves as Mormon or LDS. Again, the plus or minus for LDS data was 4.9 percentage points.

Additional findings included:

• Mormons are among the youngest Christians in America, with a median age among adults of 43, up from 41 in 2007. Only Orthodox Christians are younger, at 40.

• American Mormons are 86 percent white, but racial and ethnic diversity in the church ticked up to 15 percent from 14 percent in 2007.

• Mormons are among the most highly educated Christians, with 33 percent reporting a college degree, surpassed only by Orthodox Christians.

• In 2007, 44 percent of Mormons were men and 56 percent were women. The gap narrowed some in 2014, to 46-54, close to the Christian sample (45-55) and the overall sample (48-52). Non-Christian faiths reported more men (54-46) as did unaffiliated "nones" (57-43).

• Mormons are concentrated in the West (67 percent). In fact, 5 percent of Westerners identify as Latter-day Saints or Mormons.